Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTS), referred to as Node Bs with regard to a universal mobile telecommunication system (UMTS), and a plurality of subscriber units, often referred to as user equipment (UE) in UMTS.
In a cellular network, such as UMTS, the power transmitted by a UE is regulated in order to minimise interference with other UEs. Typically, the output power generated by the radio frequency (RF) power amplifier (PA) in the UE will vary due to any number, or combination, of factors, such as the manufacturing process, operating temperature, supply voltage, antenna loading and other such factors.
One feature associated with most wireless communication systems, which is particularly needed in a wireless cellular communication system, allows the transceivers in the Node B and UE to adjust their transmission output power to take into account a geographical distance between them. The closer the UE is to its serving Node B, the less power the UE and Node B's transceivers are required to transmit, for the transmitted signal to be adequately received by the other unit. This ‘power control’ feature saves battery power in the UE and also helps to reduce interference effects. Initial power settings for the UE, along with other control information, are set by information provided on, say, a beacon physical channel for a particular cell.
Furthermore, if an output power range is greater than that allowed by the system, then it is necessary to have feedback control of the PA to reduce the output power to an acceptable level. In this manner, it becomes necessary to measure the radio frequency transmit power at, or after, the PA output and to control a PA gain, typically by controlling the gain of amplifiers located earlier in the amplifier chain, in response to this measurement. A typical approach to PA power control is to measure DC power in the PA using a current sensing resistor. This DC power information is then used as feedback to control either the PA gain or the RF input power of the input to the PA. This feedback will allow power control regulation to compensate for variations in PA supply voltage, operating temperature & manufacturing process. This is often referred to as internal PA power control, being limited to the PA module itself. Thus, such power control limited to the PA module will not compensate for variations in the antenna loading (often referred to as mismatched load that deviates from an ideal matched load (i.e. VSWR=1:1).
In signal transmission, standing wave ratio (SWR), usually defined as a voltage ratio called the VSWR, is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line. In a wireless communication unit, the VSWR performance is important at the Power amplifier to antenna node, where reflections can occur as a result of a mismatched load or an imperfection in an otherwise uniform transmission line between the antenna and the PA, or when a transmission line is terminated with other than its characteristic impedance. For example, this characteristic impedance of the antenna may change with a body of material placed physically near the antenna. Matched impedances provide a maximum power transfer; whereas mismatched impedances provide sub-optimal VSWR and thereby reduced power transfer to the antenna.
To facilitate compensation for VSWR variations external to the PA module, a directional coupler is placed between the PA output and the antenna input. In this manner, the directional coupler routes the coupled signal in a feedback path through a detector, which generates a signal proportional to the power transferred to the antenna, as present at its input. This feedback signal can then be processed and used by signal processing logic to control either the PA gain or power of the RF input signal to the PA.
If the detector output is used in a continuous-time loop to control the PA then the question of loop stability arises. The control loop must be stable over all possible variations in PA output power. The main advantage of employing a continuous-time loop is that changes in VSWR can be compensated for very quickly. The main disadvantage of employing this technique is that the control loop performance is strongly dependent on the PA characteristics. For a wireless system, where the wireless communication transceiver and the PA may both be sourced from different manufacturers, this often creates a logistical problem, as manufacturers are often not willing to divulge sufficient information to each other. Thus, power control circuits are typically PA module specific and need to be over-designed to accommodate for variations in PA module tolerances. Similarly, the cost of supporting multiple potential PAs within a single transceiver is impractical.
U.S. Pat. No. 6,466,772 discloses a continuous closed loop control system using a gain shaper element to improve linearity. The gain shaper characteristic must be designed to be reciprocal to the characteristic of the power amplifier. Therefore, this control approach is closely linked to the PA within the loop, which makes it unsuitable for use with multiple potential PAs.
Thus, there currently exists a need wireless communication unit, integrated circuit and method of performing power control in a transmitter, wherein the abovementioned disadvantages may be alleviated.